Certain virgin and/or recycled polymeric materials, as used for molding plastic materials, are known to require treatment in appropriate plants for drying, upgrade and possibly crystallization.
Particularly, the materials selected for this purpose are polymers obtained by polycondensation reactions and selected from the group comprising polycondensates in general, such as: polyamides, elastomeric polyesters and PET and the term “upgrade” as used hereinafter is intended to designate the process of forming larger polymeric chains by repeated condensation reactions which occur in the solid state, to increase the molecular weight of polymers and, as a result, the inherent viscosity of the material.
Treatment plants generally use a plurality of treatment working stations in which a hot air stream flows through the polymeric materials to promote heating thereof to predetermined temperatures.
A first drawback of treatment plants is that the hot air that is used for thermally treating polymers causes an oxidation reaction which alters the structural or aesthetic characteristics of the material.
Furthermore, the material is heated using a plurality of distinct thermal stations cascaded along a product feed line, and this eventually increases the complexity and overall size of the plant.
In order to at least partially obviate the above mentioned drawbacks, a number of plants have been developed for thermal treatment of polymers by application of an electromagnetic field, generally in the radio-frequency (RF) band.
These plants are particularly suitable for treating polymerized-by-condensation materials, with temperature being controlled either by adjusting the electric parameters of the electromagnetic field, particularly frequency and/or power, or by adjusting the field application time.
Thermal treatment of polymers by a RF electromagnetic field has the effect of removing the water that is naturally contained in the material to increase its molecular weight and provide a high-strength, high-viscosity polymer.
U.S. Pat. No. 6,316,518 discloses a plant for thermal treatment of polymers, e.g. polyesters, polyamides and polyurethanes, comprising a feed conduit for the polymeric material to be treated and two treatment stations along the feed conduit.
Each of the stations comprises an applicator for emitting a radio-frequency electromagnetic field of predetermined power, which is adapted to heat the polymeric material to a temperature ranging from 190° C. to 205° C.
Particularly, the electromagnetic field generated by the electrodes has a frequency of about 40 MHz and its application time is of about 30 minutes per kilogram of material being treated.
Furthermore, the plant comprises means for blowing nitrogen into the treatment stations, in order to remove moisture that forms during the material polymerization reactions.
A first drawback of this arrangement is that the electromagnetic field generated by applicators is not uniform and cannot ensure optimal heating of the polymeric material that passes through the conduit.
This drawback hinders the drying and solid state polycondensing processes, whereby the polymeric materials being treated are found to have a very low molecular weight.
A further drawback is that, since the electromagnetic flux lines are transverse relative to the feed of the material in the conduit, the flow rate of the material shall be very low to ensure treatment of the polymeric material that is being fed therein, which can be obtained by reducing the diameter of the conduit.
This drawback will further increase the overall times required for thermal treatment of the polymeric material.
Furthermore, due to the particular conformation of the electromagnetic field being used, this plant has a very low thermal efficiency and a very high overall consumption.